Low notes, quiet (4)
I believe the answer is:
deep
'low' is the definition.
(similar in meaning)
'notes quiet' is the wordplay.
'notes' becomes 'de' (two musical notes).
'quiet' becomes 'ep' (I can't justify this - if you can you should give a lot more credence to this answer).
'de'+'ep'='DEEP'
Can you help me to learn more?
(Other definitions for deep that I've seen before include "down a long way" , "Extending far down" , "Main" , "Cavernous" , "Sunk low" .)